ST. LOUIS • Mayor Raymond Tucker rode in the dome car with Lilian Saarinen, widow of the architect who designed the Gateway Arch. They cut a ribbon for a new railroad tunnel on the riverfront, then returned to Union Station for lunch. It was a short, pleasant trip on Nov. 21, 1961, with hardly a hint of the 26 years of stalling, setbacks and suspicion that had to be overcome to get there. The occasion was the dedication of the tracks that still carry trains through the tunnels and sloping cuts at the Jefferson National Expansion Memorial. They replaced a dingy steel trestle that stood for eight decades along the riverfront, and which was the biggest single obstacle in the prolonged effort to build the Arch.